Carbon fibers with carbon contents of about 95% by weight or higher exhibit tensile strength of 300 kg/mm.sup.2 or more and 20,000 kg/mm.sup.2 or more. Thus, they are usefully processed into fiber strands or chopped fibers, and are used in combination with various matrices such as thermosetting or thermoplastic polymers. The resulting composites are extensively used in the field of aircraft, automotive and sporting goods. When carbon fibers are prepared by preoxidation and carbonization using acrylic fiber as a precursor, a weight loss of from 45 to 50% usually occurs during pyrolysis and the fiber production requires temperatures higher than 1,000.degree. C. in an inert gas atmosphere. This weight loss and the high temperatures used lead to increased materials and energy costs. In addition, because of the need for using a furnace adapted to operations at temperatures over 1,000.degree. C. and a special refractory material capable of withstanding such high temperatures, high initial investment costs are involved, and this results in raising the price of the carbon fibers obtained. In spite of their high cost, conventional carbon fibers, having excellent physical properties and quality, are extensively used in industrial fields where quality is a predominant factor, but not in fields where low cost is of primary importance.
Carbonaceous fibers with carbon contents of 90% by weight or less are conventionally obtained as intermediates for the production of carbon fibers and are less costly than the carbon fibers which form the final product. On the other hand, carbonaceous fibers typically have such poor physical properties that, in comparison with carbon fibers, the cost performance of carbonaceous fibers is too poor to provide an incentive for using them in many fields. In other words, if the physical properties of carbonaceous fibers can be improved, their use in cost-conscious fields currently dominated by carbon fibers will be increased.
It has been described that carbonaceous fibers can be produced by stretching preoxidized fibers in an inert gas atmosphere at temperatures between 350.degree. and 500.degree. C., or between 400.degree. and 800.degree. C., and further carbonized at temperatures higher than 800.degree. C. (see Japanese Patent Application (OPI) Nos. 147222/79 and 63012/81). However, the carbonaceous fibers produced by these methods do not have fiber performance comparable to that of carbon fibers.
With a view to improving the performance of carbonaceous fibers, the present inventors made detailed studies on the starting materials and the manufacturing process involving preoxidation and pyrolysis steps. As a result, the present inventors have found that their object can be attained by setting specific conditions for each of the preoxidation and pyrolysis steps, as well as by combining the two steps in a systematic way. The present invention has been accomplished on the basis of this finding.